Organic compounds belong to one of two groups; large molecules produced by natural processes (humic acids, fulvic acids, and proteins), or small molecules (monomers of larger particles, and synthetic organic chemicals). Complete oxidation converts a specific organic compound to carbon dioxide and water. These oxidation processes that take place are rarely complete. The more reactive organic compounds (phenols, dyes, aromatics containing OH and NH2 groups, alkenes, alkynes, and amines) are often partially oxidized to form more stable compounds, such as aldehydes, ketones, organic acids, and alcohols.
Natural waters contain varying concentrations of numerous organic and inorganic compounds. The main organic constituents in natural water are a collection of polymerized organic acids called humic substances, as well as, carboxylic acids, amino acids, and carbohydrates. The concentration of these compounds is very important in determining the ozone demand because of their specific reactivity with ozone. More importantly, it is these substances which can create taste and odors in water or possibly cause adverse health effects.
Aquatic humic substances (humic and fulvic acids) have been used to represent natural organic matter. These compounds result from the degradation and polymerization of vegetation. Aquatic humic substances are complex, their main physiochemical properties include:
The action of ozone on aquatic humic substances at neutral pH primarily leads to:
Non volatile ozonation products derived from humic substances include:
These reactions as a whole lead to a slight loss of organic carbon and a decrease in the average apparent molecular weight of the humic matter. Simultaneously, ozonation induces a distinct increase in acidity through the carboxylic functions; a formation of trace amounts of some aliphatic acids, aldehydes, alkanes, and other products; and finally the removal of color.
Micro pollutants present in water may include: aromatics and aliphatic hydrocarbons, chlorinated solvents, phenols, substituted or non-substituted polyphenols, pesticides, plasticizers, and surfactants. Micro pollutants generally only make up around 5% of the total organic carbon (TOC) in surface waters.
Ozone reacts first with multiple bonds (C=C) or atoms carrying a negative charge (N, P, O, S, and nucleophilic carbons). A strong initial reactivity is therefore predicted for ortho-activated aromatics by substituent such as OH, CH3, or OCH3. A weaker initial reactivity is predicted for molecules with NO2, CO2H, or CHO groups.
Ozonation can remove many SOC’s (biocides, hydrocarbons, phenols, plasticizers, dyes, amines, aromatics, pesticides, and solvents). This removal leads to the chemical transformation of these molecules into toxic or non-toxic by-products. It should be noted that these compounds occur at very low concentrations.